Information videotex systems, such as that developed by the Department of Communications, Canada, known as Telidon* (*a trade mark for which notice has been given under Section 9(1)(n)(iii) of the Canadian Trade Marks Act by the Ministry of Communications of the Federal Government of Canada), have been developed to provide videotex information to subscribing consumers which can be displayed on an ordinary television set or a video display terminal adapted for such purpose. Such information might include, for example, weather, news stories, travel information, and entertainment.
The Telidon* system defines a set of digitally represented codewords and syntax for those codewords creating a specialized information language which, in turn, is used to communicate to a subscriber a variety of information types. A receiver receives the videotex information, decodes it and displays it on an appropriate video display screen. Examples of information types are graphics, text, and images. Various information types may be combined to produce one display "page" of information.
A variety of communications links might be chosen to transmit videotex information. For example, a communications channel might be selected from television broadcast or cable channels, or an optical fiber link might be chosen. If a standard (occupied) television broadcast channel is elected, the videotex information is incorporated into the broadcast frequency channel in such a manner that it does not interfere with or affect in any way, the reception of the standard television signal. At the receiver, a decoder is necessary to decode the Telidon* videotex information and display it on a video screen. For any given selected mode of transmission, the decoder identifies received videotex information signals (and distinguishes them from other signals which might be received with the videotex data signals) and interprets and displays the information on a video screen.
A standard for Telidon* known as North American Presentation Level Protocol Standard (NAPLPS) has been adopted by the Canadian Standards Association (CSA) and American National Standards Institute (ANSI). This standard assumes serially transmitted data (i.e. discrete binary digits of digital codewords are sequentially transmitted from, and received on, the same two-wire pair or fiber) and defines for each digital codeword one or more meanings, the specific meaning depending upon accompanying codewords (i.e. the syntax within which the codeword appears). One or more predetermined codewords are control codewords which provide information regarding the type of immediately following information. For example, where graphics occur, the control codeword for graphics (i.e. Picture Description Information--PDI) precedes the start of graphics data so that the codewords following the control codeword are interpreted in accordance with the particular control codeword. Therefore, if the control codeword for graphics occurs, the codewords following the control codeword are interpreted according to a codeword subset for graphics. Any given non-control codeword might fall under a variety of codeword subsets, the specific subset applicable being determined by a control codeword. Graphics and video images are two information types which may be provided in the Telidon* system and for which a control codeword has been defined, each information sequence associated with either control codeword providing pictorial information describing a defined area on a video display screen.
When it is desired to deal with discrete packets of information (i.e. digital format comprising, for example, bytes of data) describing a video display picture, for reception and display by a video display terminal, it is convenient to divide the area of the video display terminal screen into a number of discrete smaller areas, known as pixels, combining to form an array of pixels, known as a video raster. Each pixel can have assigned to it discrete visual data and so, the greater the number of pixels chosen for the screen raster, the greater will be the picture resolution capacity of the selected display screen. To illustrate this, consider, for example, that if a circle is displayed using only a small number of large-area pixels of square or rectangular shape, the intended circular shape would not appear to result from any arrangement of the pixels chosen to construct a circle. However, if a large number of very small pixel areas are elected, the smaller square or rectangular areas are able to track the curvature of the circle more closely to give the visual appearance of a curved line, forming a circle. Telidon* standards for photographic image information define a pixel resolution of 256 horizontal by 192 vertical. That is, 49,152 pixels (i.e. discrete picture grid areas) together comprise the screen area used to display videotex information in the Telidon* system.
Graphic information is defined by a number of descriptive codewords: For example, a sequence of graphic codewords might follow a graphic control codeword to indicate that a "rectangle" is required; the rectangle is to comprise 50 pixels in width and 40 pixels in height; the top left-hand corner of the rectangle is to begin at pixel location 100,25 (i.e. the 100th pixel in the 25th row of the screen raster); and the rectangle is to be in the colour blue. Pictorial graphic displays are, in effect, a combination of defined shapes of various sizes, colours and positions. This method of coding graphic images is frequency bandwidth efficient and/or time efficient for transmission purposes because a relatively small number of codewords is required to represent pictorial information for a substantial number of pixels i.e. information representing a large picture area. This is so because graphic information is transmitted in a general descriptive format rather than in a point-to-point (i.e. pixel-by-pixel) format.
Photographic image capability is provided by the Telidon* system and is an information type included in the NAPLPS standard for videotex information systems. Instead of generally describing pictorial information, as in the case of graphic information, information is transmitted for each pixel of the display screen (i.e. for each of the 49,152 pixels) in a pixel-by-pixel format. Digital pixel-by-pixel image information is commonly provided, for example, by a real-time video camera. Since every colour can be represented to an acceptable degree of visual satisfaction by a specific combination of, for example, the three primary colours, red, green and blue, a colour image may be defined, on a pixel-by-pixel basis by the relative primary colour components of the discrete image area associated with each pixel. Photographic image videotex systems have been developed which transmit three codewords for each pixel, each codeword representing the relative primary colour component for one of three primary colours. Consequently, those systems require transmission of three codewords for each pixel, i.e. 3.times.256.times.192 codewords (equal to 147,456 codewords), for every photographic image provided by the videotex system.
The disadvantage of such systems is the large frequency bandwidth required to transmit and receive a large number of image codewords within a preferred short time interval. Given any specific number of codewords to be transmitted, the trade-off is always increased frequency bandwidth in exchange for decreased transmission time or decreased frequency bandwidth in exchange for increased transmission time. Neither increased frequency bandwidth nor increased transmission time is desirable for the purpose of videotex information systems. Frequency bandwidth, in a global sense, is a limited resource and, in a more practical sense, normally involves more expensive equipment the greater the bandwidth desired. Increased transmission time implies longer waiting periods to display videotex "pages", which in turn may result in user dissatisfaction with the videotex information service and fewer subscriptions to the service. Accordingly, it is desirable to reduce the number of codewords representing a photographic video display image in order to reduce the frequency bandwidth and/or time required to transmit the videotex data (i.e. the image information).